1. Field of the Invention
The present invention relates in general to a structure for coupling pipes, and more particularly to a structure for coupling a tubular pipe with a breeches pipe, which is used in an exhaust manifold of an automotive internal combustion engine. More specifically, the present invention is concerned with a structure for coupling, via welding, a cylindrically united end portion of the breeches pipe with one cylindrical end portion of the tubular pipe.
2. Description of the Prior Art
In an exhaust system of an automotive internal combustion engine, there is provided an exhaust manifold which carries waste emissions away from the combustion chambers of the engine. Some of the exhaust manifolds are of a type which comprises a plurality of tubular pipes each of which has one end connected to an exhaust pipe of the exhaust system and corresponding number of breeches pipes each of which has a cylindrically united end portion coupled, via welding, with the other end of each tubular pipe and separated two ends respectively connected to exhaust ports of the engine.
Some structures for coupling, via welding, the united end of a breeches pipe with a tubular pipe are described in Laid-open Patent Application 57-47523, which are schematically shown in FIGS. 4, 5, 6 and 7 of the accompanying drawings.
As is shown in FIG. 4, in one conventional coupling structure, a cylindrically united end of a breeches pipe 11 is connected via welding "W" to one end of a tubular pipe 13.
FIGS. 5A, 5B and 5C show the method of producing the breeches pipe 11. As shown in FIG. 5A, for the production, a straight tubular pipe 15 is used, which has a flat portion 15a formed on a cylindrical wall thereof. Thus, the cross section of the pipe 15 at the flat portion 15a is shaped to have a straight part 15b and an arcuate part 15c, as shown. Then, as is shown in FIG. 5B, the pipe 15 is cut at only the semi-cylindrical wall 15c except the flat portion 15a, and then, as is shown in FIG. 5C, the pipe 15 is folded largely in two along the cut portion.
However, in the structure of FIG. 4, the coupling between the breeches pipe 11 and the tubular pipe 13 is somewhat weak because of inherency of such coupling structure. That is, as shown, the two members 11 and 13 are mated in a so-called end-to-end connecting manner and welding "W" is applied to only the mated end portions, which fails to allow the coupling therebetween to exhibit a satisfied unifying force.
In order to solve the above-mentioned drawback, one measure has been thought out which is shown in FIG. 6. That is, in this measure, the cylindrically united end portion 11a of a breeches pipe 11 is snugly inserted in one end of a tubular pipe 17, and welding "W" is applied to a portion where a leading end of the pipe 17 is mated with the cylindrical wall of the united end portion 11a of the breeches pipe 11.
However, even this coupling structure has a drawback which is depicted by FIG. 7. That is, as shown, due to inherency of such coupling structure, there are remained certain gaps "S" between the inserted united end portion 11a of the breeches pipe 11 and the tubular pipe 17, each axially extending along the mated edges of the folded two parts of the pipe 11. In fact, such gaps "S" originate from grooves which are inevitably produced when the straight tubular pipe 15 (see FIGS. 5A, 5B and 5C) is folded into two. Such gaps "S" however bring about a higher possibility of leakage of exhaust gas to the open air therethrough. Even though such leakage can be minimized by covering the spaces "S" with welding "W", complete suppression of leakage is not achieved because welding "W" can not be applied to the entire length "L" of the united end portion 11a of the breeches pipe 11 which is received in the tubular pipe 17.